Banjo! Courage mon Brave! You'll be just fine. Pretty much everyone starts quite slowly on the Merlin - a few "enthusiasts" tear off the front but we usually see them again quite soon enough

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Most folks there will be unused to bunch riding and will give you plenty of room.
Start near the front but not on it. You'll be surprised how slow many Merlinners are - there's quite a relaxed, family atmoshere and many folks will be aiming to just get round the '50 and will be a lot slower than you.
On the first hill, we sort the sheep from the goats - take it easy like Luke'sDad taught you and let the racing snakes go. You'll probably do a great deal of overtaking yourself. Most of those riding will have less experience than you of long distance stuff - IME most of the protagonists will not have ridden 50 continuous miles before. You'll probably settle into a similar-ability group at this point and you'll start to rather enjoy yourself. You've seen the Tour de France on the telly right? Well, this is as close as most of us will ever get to a proper peleton and it's a proper laugh. Lards, yours truly and another chap pulled a bunch of 60+ riders from Hirwaun to Cimla during the Dragon Ride last year - an amazing experience. Fairweather was on the back, counting heads whilst blazing along at 30mph with hardly any effort at all!
Anyone who does overtake you - draught them for a bit if you can keep up. If they slow down a little and you're able, take a turn on the front, no matter how short - it shows that you're trying your best. I'm always grateful for a tow. Ask Lardyboy!
If you find that they're dragging you along and you can't help at all - let them go, but apologise and tell them they're too fast for you. It's the thing to do. If you find yourself dragging a bunch along and no-one helps out, ask the guy behind to go in front for a bit 'cos you need a break. He might not know any bunch riding etiquette at all and may think the really experienced chap up ahead doesn't want to be overtaken!
Don't let yourself get boxed in by riding too close to the kerb. If someone wants to overtake, make them go round you.
The guys at the front and the back of the bunch have the responsibility of being the eyes and ears of the group, hence.........
If you see potholes/drain covers etc, call out 'left', 'middle' or 'right' depending on the position of the hazard and point at it if it's safe to do so. Folks behind will soon learn why you ride the way you do and will respect you for it.
A car behind you that wants to overtake your bunch but can't because they're all over the road? Shout "Car Up!" If those in front don't get into single file - shout "Single File!" - as cheerfully as you can manage. We don't need hacked off car drivers around inexperienced cyclists. The vast majority of motorists are patient and well behaved around the Merlinners. They think it's a charity event!
A car coming head-on down a singletrack road? Shout "Car Down!" to get everyone in single file if need be.
We'll be there to help too you know.........