I'm riding a 26" MTB on tarmac roads once or twice a week. The rides are performed on a flat road around 40 kms. I take it slowly and don't push myself on the bike too much because of the chondromalacia problem I have since 10 years. My average cadence is around 80 rpms. The problem is that my leg muscles can't seem to recover even after 3 days of these rides. I feel fatigue, soreness and tightness on my calves, hamstrings and it band area. I also have some bike fit issues and still looking for the right settings. Do you think the fatigue on my legs is due to bike fit issues or chondromalacia is more likely cause for that? I have to add that my muscles are quite weak. I'm only 52 kgs despite being 174 cms tall.
1.) I am not a doctor, however 52kg at 174cm is ringing alarm bells for me. This is around 8kg lighter than a TDF mountain domestique that's spent 2 months dieting to shed excess body fat ready for the Tour.
I'd recommend speaking with a doctor (and dietician if needed). It's entirely possible poor recovery is related to chronic under-fuelling. Are you eating on the bike? Are you replacing the burnt calories when you get off the bike?
2.) 40km on a 26" mtb is a fairly big ride - you are probably doing too much too soon. Better to do a little and often rather than a lot and be inconsistent. Try 15km 3-4x per week instead of 40km 1-2x per week. You will likely recover better and gain fitness faster.
I am not a doctor, suggest seeking profession advice if you have any concerns that it is more than DOMS.
DOMS (Delayed Onset Muscle Soreness) starts after a day or two, and can last up to a week, so the 3 days is well within what to expect for DOMS. You will be sore and muscles fee stiff and tired. Light exercise, getting the muscles moving usually helps loosen muscles up, but don't overdo it and rest if it is really bad. Massage helps, I like things like foam rollers and I also use topical treatment (Menthol based creams rubbed into the muscles.)
By far the best thing is to avoid severe DOMS. In this case there is not enough explicitly stated in the question to be sure this is DOMS. The distance for a novice is quite high (even easy on a MTB, 40km is a decent novice workout). If you only recent started cycling and went straight into 40km per ride twice a week, it is entirely possible you are going 'to hard, to soon'.
You do not get fit exercising; you get fit recovering from exercise. In this case, it is possible (likely?) that you are not fully recovered before you exercise again. You are not getting the time to recover fully, and your fitness is possible going backwards.
I suggest you reduce what you are doing. If you can, drop right back - maybe 10km, and assess how you feel on day 3. Only if you are feeling recovered, should you go for another ride. If you did not get any DOMs at all, then increase to 20, and see how it goes. If you get light DOMS, make sure you are fully recovered before going out again. Focus on recovery, light DOMS is OK (even good, shows you have something to recover from), but do not do another long ride until you are feeling good.
If you have a bit of light soreness, go out for a very slow and short ride (or even a walk).
Other things to do are make sure you warmup and cool down properly, and keep hydrated.
While bike fit issues can cause muscle fatigue, I suspect the main problem here is muscle weakness and lack of fitness. You need to work on that.
I remember that in 2021-2022, I was not able to bike to work (38 km then) on two consecutive days. If I'd bike to work, I'd be so exhausted during the next day that I would take the car.
Then in 2023 winter, I started resistance training with dumbbells. If your muscles are weak, it's very hard to start this since you are able to lift dumbbells of reasonable weight only for maybe few minutes, but you should do it for few tens of minutes for it to become useful. But I did manage to improve both my muscles and my fitness using dumbbells.
Then I pumped up my tires and rode 20 km. I wasn't exhausted. The next day, I rode to work and back (43 km now), and in fact I was able to bike over 40 km per day for maybe five consecutive days.
You don't necessarily need some other training method than cycling. All you need is to start whatever training you do gradually. If 40 km is too much for you, start with 20 km, do that every day, maybe occasionally taking a day of break. Then increase to 30 km when 20 km is a piece of cake. Finally you will probably be able to do 40 km.
Usually with bike fit issues you have a general idea of whether saddle is too high, too low, handlebar is too high, too low, too near or too far away. I certainly do notice what setting is off if I'm riding a bike where the fit is off. If you use clipless pedals, add cleat location to the list of settings.
At least check first if the saddle height is correct since handlebar fit issues are unlikely to result in leg pain. You can do this as follows: put the shoes on you use for riding the bike, balance the bike next to a wall, put the heel of one foot on a pedal, push the pedal as far away (down) from the saddle as you can. Your leg should be about fully extended (assuming you are sitting straight and the other foot doesn't touch the ground but you are keeping yourself up by touching the wall). This is not the riding position, the idea is that if you have a fully extended leg in this position, the actual riding position has a leg that isn't fully extended.
If gradually increasing the distance and checking saddle height doesn't help, maybe then consider seeing a medical doctor.