I have trialled with 6/7 clubs. All adopt a convoy approach. BUT I do not want to hold others back. On two occasions I insisted that the faster riders proceed.
My experience, solely in the middle to slow end of the spectrum, is that clubs are accommodating and will wish to take care of newbies. I suggest you take an experimental approach; turn up, explain your concern and benefit from the drafting offered by the group. If you cannot hack it, by agreement, drop off.
The extent to which you preplan for this depends on the difficulty of the terrain but you should be self sufficient with water and energy bars, knowledge of local trains/taxi services/road network. Good luck.