Is it normal to stop so much because of thighs burning?
Are you using gears?
The "cadence" means (it's defined as) the number of times per minute you do a full revolution of the pedals. A good cadence if you're not wearing bike shoes is maybe 60 rpm?
When you go up-hill, your cadence shouldn't really change. Instead you change to a lower gear, and keep on with your regular cadence: use your gears so that it's constant cadence, constant force on the pedals (not pushing harder), therefore constant effort (energy output), just a variable bike speed (because of the gears); it shouldn't be much harder for you on a hill (just slower).
If you're "pushing" too hard (exerting too much force on the pedals) then again, switch to a lower gear: so the bike will slow down but, again, you keep your cadence constant.
In the extreme (if the gear it too low) you can be "spinning" up-hill: in the lowest gear, barely any effort to turn the pedals, the usual cadence, and (because you're in the lowest gear) travelling at around walking speed.
I notice your bike has only 16 speeds, maybe a road/racing bike. Don't be afraid to use a lower gear on hills, that's what they're for!
I think (I'm not an expert, this is only personal experience) it is unusual for thighs to burn. When I'm exhausted my legs just become kind of like jelly: not painful (burning), but nerveless, I can't push them anymore. And for what it's worth, before I became fitter my lungs gave out before my legs did. If the hill is sufficiently long and steep then I'm panting and eventually stop "to catch my breath" rather than because my legs are sore. I think I have 27 gears, and I use my lowest gears on steep hills (e.g. a gradient of more than 10%).
Those "compact" gears you have might be intended for "hard men" and masochists and so on (or, well, for regular racers).
Here's an estimate of some numbers by the way (YMMV of course):
First-time climbers might find hills with a 5% gradient challenging at first, but after a bit of training it will likely take a much higher gradient to create the same sort of challenge. That said, here’s a rough guide to how various gradients might feel:
- 0%: A flat road
- 1-3%: Slightly uphill but not particularly challenging. A bit like riding into the wind.
- 4-6%: A manageable gradient that can cause fatigue over long periods.
- 7-9%: Starting to become uncomfortable for seasoned riders, and very challenging for new climbers.
- 10%-15%: A painful gradient, especially if maintained for any length of time
- 16%+: Very challenging for riders of all abilities. Maintaining this sort of incline for any length of time is very painful.
Is it because I'm new to cycling, and my age?
Yes it's obviously because you're new to cycling, but maybe something else as well: a medical condition, maybe what you're drinking, or how you using your bike.
I'm about your age and so are others, so it can't be just your age.
Maybe I'm doing too much?
I became fitter with an 18 km commute: 18 km each way (36 km total), 5 days a week.
36 km is about 22 miles.
When I started it wasn't just an effort but an unaccustomed strain, and I couldn't or wouldn't do it every day. One day of work and one day of rest (or one day of work and two days of rest) might be a good beginning.
Even then I had hours of rest including lunch in the office, between each half of my daily journey.
It took me several months before I could do that every day.
The point of rest (including whole days of rest) is that apparently the work wrecks your body, and the rest (and nutrition) rebuilds the body stronger than it was before.
Now I can run longer and more often, but "24 miles, rest, 17 miles" is a significant run for a beginner.
I don't think your thighs should be burning though: I don't think I remember my thighs doing that.