I just got my bike last week, it's a single speed. I bought it to go ride the forest preserve trails near me for exercise and I don't know how far I should be riding before I'm pushing myself too far too soon. I've rode the past 2 days, 6 mile per day. Is this a good amount? Should I go further?
It depends on a lot of things, like how fit you are to start with and how well the bike fits you. The same distance can be a very different amount of effort depending on the surface, incline etc. But as a general rule (and assuming you're in decent health) tired is fine, muscle aches are to be expected, but don't ignore pain especially in your joints. As with any new exercise, do build in some rest days, and expect to build up what you can do.
If you are healthy (no heart disease etc.) and the bike fits you properly (saddle high enough etc.) you almost can’t overdo it. Bicycling is pretty easy on joints, sinews and ligaments. 6 miles (10km) sounds little but depending on the circumstances it can take you almost an hour which is totally okay for a workout.
For people who spend lots of hours per week at relatively high intensity, overtraining can become an issue. As a beginner you are probably much more limited by saddle and muscle soreness (during and after exercise) than anything else.
Make sure you get enough water and carbs during and after the ride. Get enough rest, listen to your body (a bit of muscle soreness and exhaustion is okay, real pain or dizziness is not) and you should be fine.
Except the case of some serious health issue (heart, breath, knees related ones) there is not such thing as OVER-cycling as long as you keep an "ear" on what your body has to say. Meaning if you start feeling your thighs tired, your feet sore and MOST IMPORTANTLY your knees tired (since you mentioned a single speed) there is no way to over-cycle more than over doing it as in any other sport. For example you wouldn't go running in mid summer with 45C without water etc Try to relax and enjoy your bike, don't forget to drink enough water WHILE you cycle (not just before and after) to keep your muscles hydrated and avoid cramps etc. Along with this try to maintain a more "healthy" diet and you will find yourself with the pleasant surprise of been able to cycle more and more as time passes.
Your muscles can adapt quickly, and your cardiovascular system can work on overdrive for hours without stopping. Unlike the advanced athlete, beginners don't typcally bother buying a heart rate monitor and can afford to skate on sensations (don't worry about every little thing your body is telling you).
Cycling takes a much longer time than most sports. 100 km, which is so nice to roll on a day off, takes four to five hours of clean traffic. For five hours your body runs on overdrive, your pulse is driven into an unusual training zone (you want to improve the average speed a little).