my advice would be to just get out and ride. cycling is pretty simple...the mopre miles you do the better you'll get. if you're just starting out i'd avoid thinking about interval taining etc and just get some miles in. pick some nice routes around the country lanes and enjoy it!
re comfort - your butt will toughen up. my last race saddle (pretty put just a slate of carbon fiber - no padding) took my about 4 months to break in. however now it has it is very comfortable. however invest in a decent pair of cycling shorts.....they'll make the world of difference.
Of the runs i've done so far i've always tended to start off quite easy ('dont want to get injured') and ended fast ('wonder if i can better that pb'). Only really on the 22:30 run did i try and go quick from the start.
Also they've all been in central london so hopefully i'll go a bit quicker when not stopping to cross the road etc. I had to stop to give directions to a hopelessly lost chinese tourist today!