I've restarted an exercise program, after a couple of years of not doing too much. I'm not particularly out of shape (45 yo, 5'10, 180 lbs), and can get through my program just fine, but I end up rather tired for a lot of the day - to the point where it's hard to get other stuff done. I've been biking 10-11 miles a day, at a little over 11 mph. It's a rather hilly route, and according to my fitbit, I'm hitting about 8 minutes of fat-burn, 40 minutes of cardio and 10 minutes of peak heart-rate, and burning about 680 calories.
My question is this - is it normal that after a couple of weeks of doing this 6-days a week, that I should still feel zonked for a few hours after my work-out? How long until I can do this kind of program and recover relatively quickly? Are there tricks (nutrition, warm-ups, etc.) that can reduce the recovery time?