The hardest part of a workout is starting it, and the best trick for that comes from James Clear's Atomic Habits: the two-minute rule.
The idea is to scale the habit down to a two-minute version that is almost impossible to refuse. Not "do a full leg session" but "put on my gym clothes and walk in." Not "train for an hour" but "do one warm-up set." Clear's argument is that the goal is not to complete the task, it is to master the habit of showing up.
This works because of a quirk of motivation: it usually follows action rather than preceding it. The same principle underpins behavioral activation, an evidence-based treatment for depression built on the finding that acting first can generate the mood and drive to continue, instead of waiting to feel like it. You almost never want to start. You very often want to continue once you have. The two-minute on-ramp exploits exactly that gap.
Most days, the warm-up set you promised yourself turns into the full session. On the days it does not, you still did the minimum, and that is the whole point.