Shane- I read your race report a few days ago and have been meaning to put together a response. First, congrats on finishing your first marathon, esp. given the less-than-ideal weather.
I'm a bit limited in giving you advice since we can't review your training for this marathon, seeing as you just joined the blog. But I know we can help you break 3:30 and even much faster than that.
First, your plan to rest/cross train this week is very wise. Once you can run without (much) soreness, start mixing in some short runs. By the end of 2-3 weeks after the marathon, you should be back up to full training again.
16 more weeks to train before Des Moines is lots of time. My primary suggestion would be to slowly build your mileage as much as you can- 47/week is good, but I think you will really start to see big gains by hitting consistent 60 or even 70+ miles/week (if you have the time/health for it). Doing this requires running at least 6 days a week for most people. I don't think you'll find many fast runners on the FRB who give much weight to the "Runner's World 3 day a week" type of program. It's good for beginners to help them finish a race, but doesn't help people meet their full potential (unless you want to be a triathlete with the bike/run). To run fast, you have to run a lot. There are no shortcuts, no 3-day-a-week routines. Cross training can be used to supplement your running, or once in a while in place of an easy recovery run (such as post-marathon).
As for weight training, I'm not the guy to give good advice about that. I don't do any. Some people do some. In general, I believe some solid core training can help, and limited upper body (but you don't want to bulk up with "dead weight" you just have to carry around the marathon). I'm less sold on the benefits of lower body work- some might like it, but I think running provides most or all the training your lower body needs to run fast. It's really a personal preference, so see what works for you. But I can guarantee that you will see bigger speed gains if you spend the extra time running rather than doing lower body lifting. If you have time for both, great.
Specific training advice- first, build up to 60 miles/week or more if you can. At that point, you can start to add in some tempo/speed work if you want (though more mileage is still advised if possible). Paul Petersen has introduced many of the FRB team members to the concept of Big Workouts. See this post:
http://fastrunningblog.com/forum/index.php/topic,1076.0.htmlLots of training programs have your speed workouts be rather short runs, with the thought being you need it short so you can run fast. BW are different- they have some of your longest runs be your speed ones, to better simulate what you experience during a marathon. Examples are 15 mile run with 3 miles tempo and 8x1000m repeats, or 16 mile run with 8-10 mile tempo. Very tough workouts. All other runs are E-A-S-Y, with one long run per week.
Let me know if this makes sense or if you have other questions. Feel free to ask more in the future or to check in again for guidance. Best of luck in your recovery and future training.