One way or the other it's probably going to hurt. As Ken Bonner says... he rides fast because he knows it is going to hurt, so the shorter time it hurts, the better (for him).

But going fast means you are going to hurt more than if you coast along at a lower average speed. The trade-off is that you might hurt in a different way if you spend longer periods in the saddle.

Personally, I would opt for a bike that fits me properly and is comfortable for the entire period I anticipate being on the bike at the lowest expected average speed.

From what I have learned from both randonnee riding and researching race training programs is... that you need to put in the saddle time first before you get to ride the intervals and other speed stuff.

I seem to remember that incremental increases of 10% a week in the long-distance weekend ride was in order. So if you are starting at a base of, let's say, 50km on the weekend ride, move up to a target of 55km the next week, and 61km the following week.

The essential thing is to get the endurance in your legs first. You can then start on the speed and interval training as you wish. But I wouldn't think of joining a club ride with the testosterosi until I have that endurance. Otherwise, you might get burnt and will unlikely come back when you realise you have been dropped off the back with no directions and you're totally lost... and no-one gives a damn.

PLUS, you don't have to get into speed and interval work unless you really want to. There are people who successfully ride all sorts of long distances, either as part of events or just because they want to, with a base level of fitness established by time in the saddle and with no speed or interval training.

And, ultimately, it all comes down to what you want to achieve and how you enjoy what you do. Some people get high on painful fast. Others just want the achievement and will struggle through it all. Others will just get on their bikes for a century (or longer) just because it's nice out there.