(1) Everything everyone else has said

(2) Make sure your bike is in good working order - chain clean, tires pumped up, wheel axles not too tight, etc. etc.

(3) Make sure you get enough rest between rides.
You need recovery time as well as exercise. I found my rides getting progressively harder through the week, but if I took 2 days off I would return stronger. I now ride hard on the weekends, moderate during the week and take Friday and sometimes 1 other day off during the week.

If everything should be right but isn't, think about whether either the bike or the body needs a checkup.