The main loop around tiergarten is about 6.5-7k and completely car-free. Start at brandenburg gate, head west past victory column, loop back through wooded areas

Early morning before 8am you'll barely see anyone except other runners. Well-maintained paths with water fountains at several points. Much better than trying to run along busy streets

Extend into adjoining park areas for 8-10k total. Perfect for jet lag runs when you're up at 5am anyway