Heavy legs do not always mean you need a rest day. Sometimes they are simply a sign that your body is still carrying fatigue from recent training. The important part is working out whether that fatigue is normal, or whether your body is telling you to back off.
Start by looking at the bigger picture. Did you do a hard session yesterday? Have you increased your mileage recently? Are you sleeping well? Are you unusually stressed? A single bad-feeling run is very different from several days in a row where everything feels harder than it should.
If your legs feel heavy but you otherwise feel well, an easy run can actually help. Keep the pace relaxed and give yourself the first 10 to 15 minutes to see how your body responds. If your legs start to loosen up and your effort feels normal, you can usually continue as planned.
If the heaviness gets worse as you run, your pace is much slower than usual for the same effort, or your form starts to change, that is a better sign to shorten the session or stop. The goal is not to prove you can finish the workout. The goal is to make the best decision for the training you still need to do over the next few days.
It is also worth paying attention to the type of session on your plan. There is a big difference between doing an easy recovery run on tired legs and forcing your way through intervals, hills or a long run when your body is clearly not ready. Hard sessions only work when you are recovered enough to get the benefit from them.
This is where runners often get into trouble. They see the plan as something they have to complete exactly as written, even when their body is giving them useful information. But a training plan should guide your training, not trap you into making a bad decision.
A simple way to think about it is: if you are tired but moving normally, keep the run easy and reassess as you go. If you are unusually flat, sore, struggling to hold normal easy effort or feeling worse as the run continues, adjust the session. And if you have pain, illness symptoms or something feels clearly wrong, resting is the better call.
One missed or shortened session will not cost you fitness. Pushing through the wrong session can cost you far more if it turns normal fatigue into injury, illness or several poor training days in a row.
The best training is not the training that looks perfect on paper. It is the training you can absorb, recover from and repeat consistently.