The beginning is not the easiest. You'd be astonished at how strenuously time-triallists warm up before their events, for example. You'd think they'd be exhausted, but actually they need to have gone pretty hard before they are in condition to put out their big effort. Just take it easy for the first 15 minutes until your muscles have warmed up.

EDIT. As far as RPM is concerned, pedalling faster - within reason - is better than pedalling slower. When putting out a lot of power, a cadence of between 80 and 95 is reckoned to be most efficient. But don't worry about the numbers to start with. Ride in a gear that feels comfortable and lets you maintain your momentum without putting huge pressure on the pedals (and therefore your knees). A good rule of thumb is to ride in a gear that is one lower than you feel you could manage. That should keep your cadence up to a respectable level and avoid knee problems.