As Dimitris and BDukes have said, the sensors one is using will make a huge difference here. I don't remember the actual sensor names, but the E-8 Joint STARS and one of the Global Hawks have unnervingly effective ground search radars-think being able to detect (and in some cases ID) targets from hundreds of nautical miles away (or whatever their actual ranges are). As you get to more modern tech, you don't even necessarily need dedicated recon platforms. The radar on the Rafales in the second Kashmir Fury scenario ("Escalation"), for instance, can effectively function as the primary recon asset for the Indian side-all from 36,000 feet at standoff range. I'm pretty sure the F-35's radar has similar ground-search capabilities. Granted, you won't detect infantry with these sensors, but the vehicles and structures are usually the more potent threats anyway. Just watch out for the un-radarable MANPADS!
If you aren't playing a modern scenario, of course, then you won't have the above fancy toys and you'll have to rely on more conventional recon assets and will have to get much closer to the target (horizontally at least, and vertically in most cases too). While the high-powered cameras on the U-2 or SR-71 are ideal, these platforms are generally pretty rare, and in any event are still vulnerable to SAMs at high altitude (especially the former one). By the 80s you should start looking for either FLIR equipped platforms or FLIR equipped loadouts, which can function well for ground recon at low altitude if you lack the more specialized assets. Of course, the oldest recon platforms are camera equipped aircraft. These aren't great but are better than nothing. Depending on cloud cover, they usually can spot medium and large targets from around 12,000 feet or so.
I guess the point of all this is to know the capabilities of your sensors and have realistic expectations. While you can of course use the MK-1 Eyeball for recon too, as you have noted this isn't terribly effective once you get more than a few miles and a few thousand feet away. But most of the time, a scenario that has a ground-recon component will give you the tools to conduct that reconnaissance. Figure out what they are and find a way to use them!