Sell your Garmin and buy a powertap.

Powertap is the sweetspot of price to value in power meters. All of the power meters have their tradeoffs, but there's a rough consensus that if you can't afford a SRM, the PT is the best compromise. (although you will get a few votes for the ergometer.)

Once you get a powertap, and Cycling Peaks WKO+, you'll have the data you need, and won't need your Garmin.

The PT will give you the data you need to train. You won't miss anytihng truely useful from the Garmin.