I too am a F1 fan. I have followed F1 for over twenty years and attended six of the US F1 races at Indy.
I have attended only one ALMS race, but attended a couple races from the older IMSA series in the early 90's. Those cars were spectacular!
The ALMS race lengths vary. I think 12 hours is the max length ran in the ALMS series; however the same cars (with various small changes) are used in the Lemans Series which has 24 hour races.
There are a few categories in ALMS, but Prototype 1 and 2 and GT2 are the main classes. The quickest prototypes are the diesels that Audi and Peugeot run. The GT2 Cars are composed of highly modified Vettes, Vipers, Porsche 911's, Ferrari F430's, etc.
Makes for a very interesting mix of race cars, engine types and sounds.