Yes, it's about 20 miles from the SEN terminal car park to J29 on the M25. Normal journey time is about 25 minutes, although in the rush hours it could be considerably more. Still, if you can get to your car within 15 minutes of the aircraft being on-stand you can be on the M25 within 45 minutes in total.
Out of interest, what would be an average 'aircraft on-stand to M1' journey time from LTN, assuming you have parked in the longstay car park?