Cheers for the replies.
I headed out with the A group for the first time at LtL. I had been contemplating it for a few weeks but didn't like the thought of a) keeping them back when the going got tough or b) getting left behind.
At LtL, I decided I would go out with them and if I failed to keep the pace, I wouldn't be alone as there were a couple of thousand people doing it.
As it turned out, I stuck the pace for 73 miles and finished just a few minutes after them.
I was delighted, received a lot of praise for my efforts afterwards and also some opinions on how I could progress further. One of which was upgrading my bike hence my query - my bike was the only entry level one amongst my pals.
If you kept up with them for 73 miles then your entry level bike isn't holding you back.
What was it about the last ten miles you found hard? If you were running out of energy then more training and experience will sort that out.