Running is the main training for a running event. Strength work is a supporting tool, not an excuse to exhaust your legs before every run. Start with movements you can control and a workload that leaves you able to run normally afterwards.
An illustrative home session
After an easy warm-up, try a chair squat, glute bridge, calf raise, incline push-up and bird dog. A starting example is one or two sets per movement, with roughly eight controlled repetitions and fewer if technique deteriorates. This is an optional general routine for healthy beginners, not injury rehabilitation or a marathon-specific prescription.
Use the gym without chasing fatigue
If you already lift, a squat or leg press, a light hip hinge, a row and calf raises can cover useful movement patterns. Choose loads that leave a few good repetitions in reserve. Learn unfamiliar lifts with qualified help. Heavy soreness is not a measure of how useful the workout was.
Fit it around the running week
Begin with one short session and assess how your next runs feel. Add another only if recovery allows. Avoid introducing a demanding leg workout immediately before your long run, and reduce strength volume when the running programme becomes heavier. Do not increase running distance and lifting workload aggressively in the same week.
Progress one thing at a time
When every repetition stays controlled and your running remains comfortable, add a little resistance or a few repetitions, not both at once. Stop movements that cause pain. Use the FitGeek exercise library for form cues and easier variations; recurring pain or an injury needs a clinician's assessment rather than a harder version of the exercise.