TransIowa is all about pedaling your bike at HR Zone 2 and Zone 3 for a LONG time AND staying hydrated and taking enough calories. Doing this is easier said than done because there are no aid stations or outside support. You also must be prepared for all weather conditions and road conditions. A mental mistake like not eating or drinking enough, an overuse injury or being too slow (time limits) will prevent you from finishing.
As far as training, I would recommend doing a few 5-7 hour rides in the months leading up to the race. These rides should be in Zone 3 (Tempo) or maybe a little into Zone 4 (LT). Don’t forget your speed work either. It would also be helpful if you have done a 12 or 24 hour solo race in the past or a brevet (sp), double century, or other long distance cycling event so that you know how to properly hydrate and eat over the course of a long ride/race. Long rides also prepare your rear end for 26-30 hours in the saddle.
Last year as preparation (I was a Trans Iowa finisher) I did a 12 hour solo mtb race in February and a few fast road centuries in March and April. Long rides in near zero temps and driving snow proved to be good mental preparations for TransIowa.
I am planning on doing the race again this year but my training will be a little different.