Taz & Polly - just take it easy and enjoy it!! It's just lovely being out in the fresh air and if you can get someone to go with you it's nice to have company. It is much better than riding an exercise bike - no comparison at all.
When my friend decided to join me, the first evening she managed about a mile and was absolutely shattered. (She was very overweight BTW) I said "we'll turn back when you feel like it. No pressure!" She turned the bike round there and then and we headed slowly for home.
Each time I went out I asked her to come, sometimes she did, sometimes she was too tired & I went alone but each time she went a little further. She was then doing 5 miles and I think the most she's done with me is 20 miles.
For a fair old time I was not using the gears on my bike to the best of their ability and after the male members of my family intervened, I have to admit that their lessons helped!