There are lots of ways round this. Obvious ones:
1. S/he puts in a number of miles before she meets up with you.
2. S/he (this is tedious, can I just say she?) treats the session as fartlek, running ahead, running back, and then recovery while jogging along with you.
3. Any combination of the above. When I (in my 50's) run with the students, I treat the session as my speed session of the week, and make it a "lollipop run": out, they run a loop while I stretch and recover a bit, and then we run back together.
4. I don't know that long runs are ever going to be really appropriate to run together, because her long run pace will be too fast for yours. Neither of you will have energy to spare to mess around and not put your very best foot forward for these sessions. The best I can suggest is that maybe one of you acts as support for the other each weekend, riding a bike and carrying the waterbottle/rainwear/snacks as appropriate. I haven't tried that. I will not do a long run with anyone, as everyone (no exceptions) is too fast for me. I really did enjoy the one long run I did at Draycote, running the wrong way round the lake five times while those running the ultra or the mara ran the right way round 7 times. Saw everybody, cheered lots of people on, really enjoyed myself.
I don't know what speed you're aiming at, but LDWA events are lovely, if you don't mind mud and like scenery. The trailfinding adds interest.