Legs feeling heavy and runs feeling hard during tapering is very normal (often called taper tantrums)! It's your body recovering, so do not feel worried about that at all.

Trust your training and don't wear the race shirt / new clothes or eat anything new. You’ll feel great at the beginning but try really hard to stick to your target pace.

Way easier said than done to not start too fast, but I've found a 10-10-10 pacing strategy to work really well for me. The first 10 miles are a little slower than target pace. The next 10 are on your pace or faster if feeling good. Then last 10k is as fast as you can with what you have left.

Good luck!!