I find that running is easier to fit into my routine and is ‘winter safe’ (you’ll probably injur yourself less if you slip and fall while running than cycling). My advice to any runner though (beginners particularly), get a decent size MP3 player and load it with tunes (I currently use an 8gb Sansa clip which I cant recomend enough, its tiny, doesnt bounce about when I run, and clips nicely onto my pants/shorts). Although funnily I run faster without music, it does help to make the runs go quicker when I just cant be arsed.
session training is good and will definitely help your fitness, but I find that going out for a run and doing 10x hill repeats feels too much like training so I just go out and run and forget about everything else other than running. Running alone is good but some people prefer to run with other people and I am sure there are running clubs near you, and these usually cater for people of all fitness levels.
Muddypuddle, I used to suffer with achiles tendonitis, but have gone a long way to preventing it with decent trainers (my current favourites are Nike Vomero 3/4, but your requirements depend on your foot type) and some long, slow, achilles/calf/lower leg stretching; Stretch first thing in the morning, after work, before bed, hell you can even go have a stretch at work, although if you decide do it in the toilets just dont make any grunts or groans just incase your manager is in one of the traps and recognises you (although your manager recognising your grunts and groans would worry me unless you’ve had a few ‘promotions’).
The Runners World forums will probably give you more advice than anyone on here can…