Even though trail running is often associated with alpine trails high up in the mountains, this doesn't have to be the standard. Trail running works just as well over hill and dale in a small wood or on local country lanes. Even a small incline can give you many metres of elevation gain - just run it several times.
If you don't have much time, even stairs are a wonderful training environment with lots of metres of elevation gain. Training uphill increases your cadence and strengthens both your muscles and your cardiovascular system.