Worth having a think about what you might need it to do, which depends on things like how long “longish” is. For some people that might be six hours in the saddle (or not even that much); for others it might be four times that.
“Comfy” IME/IMO is 90% fit, 5% tyres and 5% fork. So: If you’re towards one extreme of the size charts then geometry’s a big factor in getting good fit, but if you’re in the middle of the bell curve then you’ll have more freedom.
And make sure that what you buy will be sufficiently adaptable in terms of tyre size: I’d suggest allowing up to 42mm at least to cover you for rides which aren’t purely on tarmacked roads (though going for disc brakes will help you here), even if most of the time you find yourself preferring something smaller. And don’t forget to allow additional room for mudguards: you don’t want hour after hour being constantly hosed with cold, oily water.
Think about luggage. Long rides often mean carrying more food/clothes/etc than you would want in jersey pockets. You have the options of saddle bags, post packs, rack packs, bar bags and more. Some will work better on different frame sizes, they all have pros and cons in terms of effect on handling and ease of access/removal/etc. And if you’re anticipating potentially increasing your luggage requirements then it may be worth considering low gearing options such as a triple.
Also consider lighting, which is impacted by luggage. Many audax-style riders will use dynamo lighting, with a front light mounted on the fork crown (which necessitates a hole in the front of the fork crown and, on smaller frames at least, has implications for bar bags or front racks); and a rear light mounted on the mudguard or a rack (for which of course you need a rack), though you can also mount them on the seatpost (implications for saddle bags, rack packs and post packs; again more so in smaller sizes).
There are loads of great bikes around, but the devil’s in the details.