Hello,
Leg length discrepancy can be concluded only after a detailed Musculoskeletal screening done by a sports physiotherapist, a patient can feel it only when it is more than 1.5 cm shortening.
Generally short leg/long leg can cause mostly low back pain not a thigh pain.
Suggest you to keep your self diagnosis (leg length discrepancy) a side and focus on thigh pain as if it is a muscular pain.
Next Steps
You must do foam roller self myofascial release for your quadriceps and ITB followed with 1 min quadriceps stretches before and after your run for a couple of weeks. Just do it as a test dose and watch out the response.
Health Tips
If your pain is not subsided or aggravated advice you to consult a Sports Physiotherapist for a screening and pain assessment.