Share 'Newby choosing a bike'

I know this is a common question, so here are some specifics. I live in Lake Tahoe and ride a mountain bike. A long time ago I was an Ironman triathlete. I am 46, in decent shape and hope to get in very good shape again. Cyclocross just caught my eye and I think it would be a lot of fun. I will ride mostly for fitness, longer rides on fire roads, some single track, etc. I'd love to do some racing…