Re: Double bass-speed - struggling, pain
Maybe its also a good idea to devide the practice sessions into blocks of 30 or 40 minutes and then walk around for a few min (get a drink, wc, etc :P) and continue. Instead of 2-3 hours straight.
For the pedal.
On most pedals the linked part (so your left pedal) usually has a bit less response then the one that's direct on the bassdrum. One way to keep this to a minimum in to keep the bearings of the axis in between the pedals clean and lube them with something like WD40.
Sometimes a slightly higher springtension on the left beater might also work to get the same response out of both pedals. Best is to experiment around and see what works for you!