Creating my own duathlon

Hi all,

I've been training on a stationary bike for a while, and recently took up running rather more seriously after discovering I can actually do it again. I had an operation for endometriosis two years ago and it's all been pretty much downhill both weight- and fitness-wise since then, as I wasn't able to do anything properly because of the pain. Thankfully, six months of a slightly drastic hormonal treatment gave me a bit of a break, and I was able to start up again.

I am not spectacularly good at either discipline - current times are 15:14 for 10km on the bike and I ran 7km in 54:54 yesterday. To mix things up a bit I quite fancy setting myself a challenge involving both, and noticed that a 'duathlon' is run-bike-run, which would seem to suit me very well.

What I don't know is what sort of a challenge to set in terms of distance and time. I know I won't be going very fast, so there will be plenty of room for improvement whatever I do, but I need to be realistic about it. I also don't know what ratios I should be using - there's no real consistency in the ones I've found details for. Some use running legs of identical lengths, others have a sorter second leg, and so on.

If anyone out there has any experience with this, and has some suggestions as to what I should start with, I'd be really pleased to hear from you!