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!
Some background....I'm 35.....I attempted a marathon twice about 5/6 years ago. Got injured in training both times and just jogged round in 5 hours both times. Then i got into cycling...
Over the past 3 years i've done quite a few cycling sportives and aim to get out on the bike at least once per week for a longish ride (30-70 miles). Until i was busy at work more recently I'd head to the gym maybe twice per week and do an hour of intervals or what not on the spin bike. No running at all during this time.
Due to being short on time at work i've recently (the last 2 months) i've started nipping out for a jog maybe twice a week round the streets of london. After tracking these runs on Strava, my best 5km time so far is 22:30, while my average over the last 7 runs is 24:24.
What d you think it would take to get it under the 20 minute mark? Realisticvaly i'm not going to be able to run more than twice per week but a little part of me is wondering if i could get under the 2 minute mark...