Well sh.. son, that would be pretty maintaining those speeds. Realistically, unless you are some kind of "Soldier" aka Kurt Russel movie, these kind of speeds would be,... difficult. Unless of course you go with an e-bike. For instance, on an all out give er' sh.. ride of 18km I'd average 25kmh. Which is 15.53Mph. At the end of it my wife would need to resuscitate me with a cold beer.

EDIT: The above is based on a road/gravel/mud/road/hills/downhills ride on a hybrid.

In terms of bikes, then in all honesty any gravel or decent hybrid will do what you want it to do. I'm a big Giant fan, not going to hide that fact, but, their Toughroad is pretty decent, actually, Cannondale's Quick series are pretty looking machines too. It depends what you like, what fits and most importantly what suits in your budget.

EDIT: I have found drop bars when riding gravel to be twitchy and awkward. Friend of mine has a gravel bike and I prefer my modified hybrid with flat bars for anything off road. It has better leverage and stability. Personal thing, just thought I'd add.