Prop A is not a bond so there isn't any borrowed money or interest. It's more Pay as you go. The first few years will be spent on design and engineering which will allow the fund to build up money to pay for the actual construction. Then you have the project like the Green Line which aren't even scheduled for work until after light rail opens when the money will be available. The cheaper improvements are scheduled 1st like Red line improvements and the Bus Rapid routes, park and rides etc.
This is a really good plan IF you like mass transit. If you don't then vote against it but anyone who says the plan is not right for X reason is full of shit or lying. The best comparison I have is Seattle's light rail. They also have a tunnel in downtown that helps the system tremendously by avoiding clusters, not taking away lanes for buses, and allowing the trains to be extended in the future which we could not do at street level due to the shorter length of our downtown blocks.