muscle pulling ;o)

[Student Bob]

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.

 

[Panza]

I am pretty sure it is the difference in how you sit on your mtb and road bike.
I have had almost the same issue, but when sitting differently on my roadbike during a race than during training. I.e elbows on the handlebars during a race where I was doing a lot of 'towing'. I almost never assume that position during training and never for over an hour like in a race.
My advice:

1. Get the same riding position on both bikes.
2. Do weight training
3. Stretch