Don't be obsessed by a high average speed. Enjoy the ride, listen to the birds, watch things happening around you, stop and look at the view. Seems to me that the whole world is in too much of a rush. BTW my average over 39 miles and 2,100 ft of climbing was 15.2. Am I bovvered?
That's the best plan, if you're not racing, or planning to race, why worry, just enjoy riding a bike, even turn any speed/distance recording stuff off, or leave it at home, ride your bike, slowly build up the distance, you don't need to record things to add another 5 miles to your ride, and if you're not planning to race, don't even think about about interval training, it's very painful, and no fun.
There are so many advantages of riding a bike, and 30 miles at 16mph, is unlikely to be of much more benefit to mind and body than 30 miles at 14.5mph, however 30 ish miles in around about 2hrs, or perhaps 50 ish miles in about 4hrs will probably be a lot more fun, especially if you can add pie and chips, and a couple of pints.
When I stopped racing many years ago two things changed in my life, I suddenly had hairy legs, and riding a bike became more fun.
I'll head out for a wee ride this evening, take my hardtail, probably plan to do about 30 ish miles on the lanes, may divert to a few tracks and paths if I feel like it, and I'll probably be out for a couple of hours, I may record my ride, I may leave my Garmin at home, it really doesn't matter, riding a bike is fantastic, just enjoy.