So, I'm training for my first Tri. It's the last weekend of August. Unfortunately I've gotten to a slow start. I was in PT for my back for 3 months and then I badly strained a hamstring.

The race consists of:
1,000 m swim, 14 mi mountain bike, and 6.5 mi trail run

My questions are:
How often and how far do my bricks need to be?
And are "bricks" just the combo cycling/running or do I swim/bike too?
Is it okay to do road rides and road running as a substitute brick?