well ive exercised from day 1 but only like twice three times at the gym per week..whilst i was away i completed two bikes rides..one of 21.5 miles and the other of 7.5 and whereas i dont usually have a problem and can do a lot more than that, i did find i got very tired towards the end of the larger ride...id say pace yourself and if you feel too tired or lightheaded just stop for a while
good luck hx