Its both legs .... Right first then the left follows a bit after
The fact it's both your legs tends to suggest that it's a 'central' problem - i.e. pain referring from the lower back and could be any number of things!! SI joint, referred pain from elsewhere... It's hard to say much more without seeing you on the bike!!
However, it does sound like a simple musculoskeletal thing brought on by the increase in mileage, but it's well worth making sure that your saddles are at the right height (Is it 107% of your leg length??) and getting some stretches in.