Long runs

I'm aiming to do a particular race in the States in October next year,and one of the challenges they have is to run a half marathon and marathon on consecutive days over the weekend.

I was just wondering how to prepare for this, particularly regarding long runs etc. I aim to run under 4.5 hours in the marathon.

It sounds like you want to do your long run and a medium long run on consecutive days in training. So you could (for example) build up to running 12 miles on Saturday followed by 20 miles on Sunday.

You would then shuffle the remaining training days around to suit, with Friday and Monday being rest days, or the easiest days of the week if you are running more than 5 days per week.

I think with a lot of these things, you need to weigh up specificity of training for the event with the need to progress from what you're used to.  In other words ideally you want to get used to running fairly long efforts in training on consecutive days, but that's not something you want to jump into if you're used to having a rest day either side of your long run.

During normal marathon training I would do my long run on a Sunday but Saturday would also be a quality run, either long intervals or maybe a tempo run.  For consecutive HM/marathon I would probably adapt the Saturday run to be more endurance orientated but still with a bit of quality, e.g. progressive run with a fast finish (marathon pace or close) or with marathon pace in the middle.  So I'd be quite happy going over HM distance on the Saturday but still doing a 20m+ run on Sunday.  If you're not used to such a heavy work load and/or quality back to back, ease in with gradually increased mileage on the two consecutive days before adding any quality.  And have a rest day either side. 

Thanks for all the advice so far guys. Glad I've got long enough to build up for this

. Another factor I'll need to take into acount is that the half is reasonably flat but the marathon has quite a lot of 'rolllers' !

 Just glad the South Downs and Ashdown Forest are close by for training!