Hi Ian!
I have trained for four marathons using the FIRST schedule and it works really well for me. The down side is with each session being a 'hard' session, running can become a bit of a grind/burden. But if you are doing an easy session anyway, this could be a 'just for fun' session.
Which FIRST schedule are you using, the first time marathoners one, or the getting quicker one? I used the improvers one (even for my first marathon), but I had been running for a couple of years and had a good running base.
I didn't do any cross training for the first two marathons because time was too tight. I still got GFA qualifying times. Third time round I did 10 more miles a week as an easy run, and it did seem to make a difference, but I never got to test it out as I had to pull out of my third marathon 3 days before, after a nasty fall.(NOT to be recommended: I had done every run on the schedule!) With my fourth marathon I did four runs a week, again one extra easy session, , but barely got above 40mpw, and this was my PB, of 3:32 at this year's FLM.
I am a 44 year old female, and don't think I am particularly gifted at running. But I do have the discipline to stick with a schedule, come rain, wind, or dreaded mile repeat sessions!! If you are good on self discipline, I am sure FIRST will work well for you too.
I think the main thing to be gained by the cross training is having a better aerobic base. If you already run a lot of miles a week, you should be beginning from a good starting point, and FIRST should just build on this. The LR is fairly fast by a lot of peoples' standards, but is quite manageable with the speed you get from the LT session and speedwork. Just make sure you have a day or two off afterwards, because not getting to the start line is a common marathon training problem.
I can't comment on the book, as I've not heard of it. I think an excellent aerobic base is important for marathon success and a lot of people get this by a lot of slow miles. A somewhat smaller aerobic base , but more long repeats and working on increasing Lactate Threshold (as per FIRST) clearly also works well for marathon training. Combining the two is probably what the elites do, but would probably mean the average mortal died of fatigue/injury long before race day!
Edited: 19/09/2008 at 19:43