Prsumably it is an AA flight. If so -- you CANNOT do it. You have to make it form your flight to flight connections (which requires a security screening), clear flight connections (since your return boarding pass will not be issued in DFW), and make it back to your gate. And since flights close 1/2 hour prior to departure, you CANNOT make it.

I would leave yourself at least 1-1/2 hours in LHR for a mileage run turn.

(For reference, last year I did a mileage run to LHR and left 2-1/2 hours between. The outbound flight was two hours late and landed at 11:30 for a noon departure. I was told by the groud crew that the noon departure was closed and I would have to wait for the next flight (luckily, the EXP desk rebooked me automatically so it was not that painful)).

--Jim