The number one fault rookie marathoners make is going out too fast. The world is full of first-timers who felt fresh until the 18-mile mark, then had the hammer crash down by the 20th mile. So it sounds like your training partner is actually looking out for your best interests.
Having said that, you may want to broach the idea of a possible parting point during the race. For instance, plan to run together to a specified distance, perhaps 20 miles, then assess respective conditions. (One of you may have to forge ahead in the last 10k without the other.)
It's a long road from start to finish and many things happen along the way, so your partner's experience may be an asset for you. But the likelihood of running comfortably --at a shared pace-- all the way is not very high, unless one of you is willing to forfeit a faster finish time. Thus, a contingency plan may be the way to go.
Good luck, and smile for the camera at the finish.
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