Your body has to adapt to the physical jerks, and that takes time! In general, riding 2X/week will only help you to maintain your fitness level. When doing 3X/week you start gaining fitness. There's a corrolation between time and physical impact, eg. lets say 60 min equites to 50%impact on fitness, it doesn't mean 120 min = 100%! Optimum time to start with per session is +- 40-60 min. Also important, impact starts deminishing the day after training session, to that extend that after +-36 hours you've lost nearly everything. Bottom line, 1) start by doing 40-60 min at a comfortable pace 3x/week, spread over the whole week. Soon you will find it very easy, then you 2) start going an extra day until you can do 5days/week (now you've settled on quantity). 3)When doing 5days/week comfortably you start going a bit faster 2x/week (you start working on quality) until you can do this for each of the 5 days. 4) Next step, go 1 day further than the others. Imperative, write everything down in a log book (Excel works fine!). Set yourself a target, and more important, if you don't enjoy it, don't do it!