The marathon allowance is just a guide and if I remember rightly was only brought in a few years ago when it was the WMRA Champs - I don't think it was expected to stay.
Mind you, it was the same with the £20 entry fee :O (wasn't it £7?)

I've never done a marathon, but based on my clubmates, I'd be 3:10 - 3:20 (I've done a 1:28 HM) but my PPP is 4:39 and 4:45 at my 2 attempts. I think I have a 4:15 in me, but you have to learn the race - in particular Whernside which has been my weakpoint - and also not to run too hard on the PG to Ribblehead section. It does have great running, but better to be 5 mins slower than 5 mins too fast at Ribblehead.

I would advise the fell race qualification route though. It's unlikely you will get lost or isolated in the PPP, but not impossible. But doing some of the qualifying fell races will prepare you better than a marathon.