I had a similar issue last year and I really think that intervals helped. Same as Indy in flat-land and I was going camping in hilly country and wanted to ride while there. Intervals into the wind really helped in particular.

One thing I want to caution about parking garages - there tends to be a lot of oil from cars on the pavement and it can get REALLY slick, so be careful. The oil can also make its way to your rims to impede braking power. (I once had a pretty bad slip and fall in a parking garage from engine oil on the pavement... walking, not riding... so I'm cautious even when walking in parking garages.)

Good luck with your training!!!