What discouragement ???? The only thing I saw was people suggesting the OP know his / her limitations and what he / she might find comfortable for a first tour. Sorry but without knowing the age and / or physical limitations of the OP, suggesting they CAN do 75 miles or whatever the above suggestion was borders on irresponsible.

A 50-60 mile day is far more reasonable and probably within the reach of most people but the only one that will know for sure is the rider.
I think that for a first tour, it is far better to wish you did more miles rather than regretting you have exceeded you limit and ruin the whole trip. There is always another tour out in front of you. Better to want more then come away hating the thought of another tour.

And you give excellent advice, you do need to prepare no matter what mileage you decide in the end.

With those new parameters, I would say you have nothing to worry about. That seems like a very reasonable pace for someone so young and fit. My advice it don't fixate on the miles one way or the other. The most important thing is to enjoy yourself. Some people want to do lots of miles in a day, other prefer to take a slower pace, take time to smell the flowers.

Both are excellent choices and up to you. And nothing says you can't do both. You are young and will have lots and lots of tours ahead of you. Maybe you do one tour where you take your time to see the sites, talk to folks along the way. Then the next tour you set a goal to be in a certain place in a specific amount of time with lots of miles per day. There are lots of choices.

And be prepared. Do you training and do that overnight I mentioned. An overnight really helps to shake out the buts of what to take and what to not take. How to pack etc.

This is great advise, as are all of the other posts. Thank you for all your positive and helpful posts back, this forum is awesome!
I am thinking I may be doing a 60 mile ride to see my friend in Brighton so that should be good fun and I will see how I get on with that.

Thanks guys!!

It sounds to me like you will be fine. I'd suggest trying your commute with a fully loaded bike a few times though, if only to get a good idea of how much of a difference in your speed the gear will make. And it really does make a difference. My own experience is that for a given amount of effort, loaded panniers can easily reduce my rolling average over a whole day by 2-4 mph. You might also find the need to change things about the bike's setup to make the load more comfortable, too.
But if you've been riding 10mi/day and commuting 46mi once a week, my guess is that you could load up your bike and start tomorrow and you'd be fine.

My one caveat is that if you really are going to try and average ~38 mi per day (and presumably avoid many days that are very much longer than that) it's going to be a lot harder to plan your overnight locations because you will have to find so many of them close together, but evenly spaced. In some areas it's fine, in other areas you might end up needing to do, say, a 70-mi day followed by a 20-mi day or something. But that's not necessarily a bad thing, either. Sometimes it's rewarding to do a longer, harder day and follow it with an easy rest day where you can lounge around, indulge, do something else. One of my favorite experiences from a tour my partner and I did a few years ago was that after about six long, hard days (mostly 90mi+) through the Rockies, we had a rest day where we rode an easy 20 mi in the morning, then spent most of the day soaking in the hot springs, then another easy 20 mi to our campsite with a big meal halfway through it. The day after that had the highest, longest climb of our entire trip and it was really nice to start it fresh. I so wish we had hot springs near where I live!!!

Another factor I didn't see mentioned is "mind set." For myself, it is one of the most important factors.

Last year I did my first, albeit short, tours. Prior to that I had never, not ever, rode more than 52 miles in a single day and never two days in a row. I did do a lot 25s 4 to five times a week though. First trip was 200 miles one way and my excuse was to see the grand kids. Second trip was 220 miles one way and my excuse was to see a sister I hadn't seen in years. Both times I found it was relatively easy to just keep plugging away. Longest day was 76 miles and I could have gone further. Some camping, some motel. BTW I am 68 y.o. and did it on a bike that most wouldn't consider a "touring" bike.

Go for it. Pack what you think you will need then cut it in half. Jump on your bikes and head out. You'll almost certainly have a great adventure.