This is really just for my records - but might be helpful for someone else starting out as old (early 50s) and unfit as me. Could have been even worse though, I suppose, as I had already been doing a fair bit of walking before I started.
I went out and paced out 100m and timed myself to walk and run. My brisk walk was exactly the same pace as my very gentle run - both took 55 seconds, which would mean 45 minutes for 5k. (But over the whole 30 minutes yesterday, my average pace was slower even than this - it would have taken 53 minutes for 5k.)
It's all activity though, and for me, a good place to be starting. I think I found the first attempts so hard because I was going faster (44 seconds for 100m) which at the moment is too fast for me (but if/when I can keep it up, it'd take 36 min for 5k - just about ok for a first parkrun around here, at the very back of the field).