Is there an android phone app or something similar that will do structured workouts similar to the Garmin 520/820?

I'm sure there may be better options than what I've got here but two that immediately come to mind are:
1. Kinetic - this is the app that works with my trainer. It's free and worth every penny (i.e. it's pretty mediocre).
2. Zwift - only beta version on android, but it seems to work fine based on youtubers showing demos. Of course, this costs a fair amount, so I'd say look elsewhere unless you want to have the full zwift experience. You can probably find a cheaper way to just do structured workouts.

I'm pretty sure you can program structured workouts in apps like Cyclemeter and IPbike. I just haven't done it.

Thanks! I'll check those out

I use the Wahoo Fitness App which is free and works everything I think.

Does the wahoo tell you what interval to hit at what time?

No. You control the intervals. Do you want the software to control the intervals? If so, is there a reason you want the software to control the intervals?

Manual control allows you to add/extend/split intervals as needed. For example, if you have another interval in you, you can add one. Similarly, you can preserve workout quality if, for example, you know you won't be able to complete the second interval of a 2x20 then you can split the second interval into a 2x10 to still get 40 minutes at intensity.

You can still follow a plan since intervals are easy to control manually. You just have more control and don't have to pay a subscription.