Hi Jules, sorry to hear about your plaguing injury
My first thoughts are to ask if you've seen a physio? Obvously that's the ideal way to find the route cause of the problem. I only ever tend to find my inner thigh muscles get tired if I've been (attempting) horse riding.
It sounds like you perhaps have a muscular or biomechanical imbalance that's causing the inner thigh to be used where it's not intended to be? I may be way off the mark though, as it seems a very strange one because I wouldn't have thought the inner thigh muscle would really do anything (except stabilisation perhaps?) whilst running.
Whatever the cause, I hope the twinges don't escalate any further. Best of luck in your upcoming event.