Now that I've had some time to reflect on my spring racing season (which included the Shamrock Marathon, Gansett Marathon, and a 5K), this is my Top 5 list on how to survive back-to-back marathons.
1. Be smart. Yes, you need to get back out there quickly, but you also need to give yourself enough recovery time. Don't push it too hard.
2. Listen to your body. If you feel OK, keep training. But if it doesn't feel right, stop. Immediately. Don't wait until things get worse. If you need extra rest, take it. If you need to modify your plan, do it.
3. Learn from previous racing experience. You should know how quickly you recover both physically and mentally, how you like to train, etc.
4. Try it with shorter distances first. Race on successive weekends, or schedule half marathons a couple of weeks apart.
5. Plan in advance. Ideally, you want one race to be your target and the other to be practice or just for fun.
Did my back-to-back plan work? I'd say yes. I danced with the devil and came out OK on the other side. I got back-to-back PRs in both marathons (although part of this was sheer luck in getting perfect weather both times). I did not get injured in 22 weeks of training. I recovered quickly from both marathons.
I essentially ran the same time in both marathons, although I ran two very different marathons. My current PR is from Gansett, the second unplanned marathon. Although Gansett was slightly faster, Shamrock is the marathon I will remember as "better." I ran more evenly at Shamrock and had a physically and mentally better race. Plus Shamrock is my first BQ marathon. At Gansett, I had issues with my Garmin and knew I went out much too fast. But where better to test yourself than at a race like Gansett? You don't know unless you try. Sometimes you don't get it right. I do not regret going for it. Shamrock may have been the "better" marathon, but Gansett earned the bragging rights. I ordered a custom race necklace for Gansett, in part because it is Gansett and in part because it is my current marathon PR.
For the record, I do not advocate running back-to-back marathons. Definitely not for a first marathon. Yes, I have done it twice (2 in 10 weeks last year and then 2 in 4 weeks this year). Yes, I survived. Yes, I got 2 PRs out of it. But yes, what I did was admittedly foolish and risky. Very very risky. You really have to know what you are doing. I cannot stress that enough. You need enough running experience to understand your mind, body, and motivation. You have to cross your fingers and hope you don't get injured or burned out. I didn't have the luxury of planning in advance because I didn't know that I'd be running a second marathon until I finished the first one. I had Gansett on my radar, though. Once I had a qualifier in hand, I knew I needed to recover quickly and push only when it was appropriate.
Of course, I was highly motivated to qualify for and run Gansett. That made a difference. And it wasn't the first time I had done back-to-back marathons. The first time, I was not as good or as smart a runner as I am now. That first set wasn't pretty, but I learned from it. That made a difference, too.