First, to be clear, I have not done multiple IM distance races in the same year. I've raced IM and then an open mary 2.5 months later. My observations from the 6 IM's, numerous HIM's, and 1 open marathon:
- Coming off every single IM I've done (1st one in '91, last one in '07), after several weeks of recovery, I have always felt stronger - swimming and cycling sooner than running b/c of longer recovery period for me on the run.
- Same holds true for the HIM's
- The open marathon beat the crap out of me much worse than the IM marathons. Open mary time was 2:5x, IM mary times are typically 3:2x-3:4x.
- Where I see the biggest gains in fitness is on the bike. Post IM/HIM recovery, I'm riding 1 gear bigger than pre-race. I attribute this gain largely due to my overall training mileage/hours being on the low side (avg. ~10-12hrs/week in an IM year).
If I could swing 2 IM's in a year financially, I would. The first IM would be used as an overall barometer of fitness and used to dial in pacing and nutrition, and apply learnings to training after recovery period leading up to the second IM, what I would consider my "A" race for the year. This approach has made me consider doing a "solo-Man" training day in the Spring to prepare for a late summer IM (IMWisc or IMKy). But I'm still several years away from the next big race.
Good luck at Louisville and with preparations leading up to it!
It took awhile, but I finally discovered that its not the destination that's important, but rather the journey.