Wahoo Fitness is a good free app, with the advantage of interfacing seamlessly with Wahoo hardware. I've been using a Tickr for a couple of weeks. At 61 with a few nagging but relatively minor health issues, I figure a heart rate monitor is a better value than a power meter for now.
The recent update to Wahoo Fitness caused some glitches in both Android and iOS but I'm sure Wahoo will fix it. The previous version of the app was the most reliable and lowest resource activity app I'd used. The update is roughly comparable to Strava, which is generally reliable but glitches just often enough to be annoying.
I use cheap bike computers because all I need/want is current speed, average speed, distance and time elapsed. The odometer feature is semi-useful. Good bike computers cost from $12-$100, depending on features. The higher priced models may include wireless transfer to Strava, and at least one even features its own built in GPS, so we don't need to rely on our smartphones alone.
Even after almost four years back in the saddle and in pretty good shape for an old guy, I still don't feel much need for a pricey Garmin or power meter info. Monitoring my heart rate and subjective feel tell me pretty much all I need to know about my fitness. Joining an occasional semi-fast group ride of like-minded older dudes/dudettes tells me whether my subjective impressions match my actual fitness. If I can hang with the B-group's 16 mph average over 25-50 mile rides, I'm doing pretty well.