went out with the club for the last winter training ride, in the preceding weeks the longest ride was 60miles. Felt good, did some turns for the first 50miles, at which point dropped to the back and sat in, After the halfway point speed picked up a tad, started grovelling to hang on, at 80miles my legs cried enough. I sat up then rode a further 20miles solo, not particularly fast for the last bit but did the 100miles in 5hr:35min, with 4 hours for the first 80.
Point is, you will be fine, just a matter of getting your head to believe you can do it, and you may have to accept that the last few miles will see you grovelling a bit. Just eat and drink. If you can find a group, take a tow it will make your life a bit easier and save your legs for later.