You can discharge your battery much deeper than 60%. That rule is for lead batteries. It does affect the battery to discharge deep enough for the bms to stop the discharge, but it affects lifespan less than you might think. Do avoid hard riding when the battery is on its last 10-15%, by then ride it home slow. But yes, a milder use patter helps lifespan, ideally you don't let the battery sit fully charged for long periods in summer, and planning to stop at about 20% left is good, simply because it gives you a reserve for bad weather.
What deep discharges do is throw the battery balance off, so that its likely the next charge does not completely fill all the cells. Again, to fix that, charge, wait half an hour, charge again till it holds its full charge after a half hour. Full will be less volts as the battery ages though. Even new, its not going to be what it theoretically would be for long. Repeat the charge to balance your pack if it drops more than a volt after a half hour.
What shortens lifespan on lithium batteries is very high rate discharges, and lots of time in hot enviroment when fully charged. If your battery gets more than just warm on the discharge, back off your throttle and ride with less watts. It will get very noticably warm normally, but if its at all hot, that is very bad. If its hot where you live, nothing you can do about that. I mean if your battery is getting to 150f, that is bad. If possible, charge full and then ride it immediately in summer when its hot. Some charge to 80% in the evening, then finish off the charge in the am when they ride. But all this might be impossible. Don't make your daily routine a huge pain in the ass. Just charge your bike and ride it, and if that means its in the afternoon heat charging outside, it just is what it is. You bought this to use it, and if you try too hard to save it, it just degrades anyway due to the normal enviroment of the world being warm.
When winter comes, do charge it full, and store it where it is cold, but not frozen. Charge it monthly when its cold. Don't charge a frozen battery though. Full and cold, not such a big deal. Full and hot, its slowly degrading.
To guess range, your wattage, say 250w x one hour riding = 250 watt hours. Theoretically you have 1000 wh. 48v x 20 ah is 1000 watt hours. I'm guessing you have 800-900 actual, usable wh. So you should have at least a three hour ride at 250w average.
Get a watt meter. The blue one on ebay is cheap, and it can greatly help you understand what your batteries real full capacity is in wh, and what your actual real world wh/km is at various speeds. Once you know your real capacity, you can calculate the average wh/km you need to make a certain distance. When I did my long distance riding, my magic number was 25 wh/mile. I was packing enough battery to make it across 70 miles of desert with no house with a plug, no water, nothing. So I would keep my average pegged at 25, and know I'd survive the ride. By the last 20 miles, I would know if I could finish the ride faster, or need to pedal harder.