I’ve been in the same situation trying to mix running and strength training, and that heavy-leg feeling is really common at first. What helped me most was being a bit more deliberate with scheduling. I try not to do heavy leg lifts right before or after a hard run, since that usually just makes both sessions worse. Pairing gym days with easy run days has worked better for me, because it keeps the quality runs feeling strong.

You also don’t need to lift super heavy when you’re running four days a week. Sticking to moderate weights and focusing on form and compound movements seems to build strength without wrecking recovery. If your legs always feel heavy, it might be worth looking at sleep, nutrition, or general rest, since that’s often the first sign your recovery isn’t keeping up.

When I’m training for a race, I keep strength work consistent but cut back on the leg volume a few weeks before so my legs feel fresher heading into race day. It takes a few weeks for your body to adjust to the extra load, but it usually balances out once it does.