Historical buildings are a great source of real floor plans: guild halls, merchant adventurer hall, churches, and castles will all have a visitor's map which you can easily adapt to your game.
Another great source is archaeological digs of pre-dark ages sites: Carthage, Rome, Greece, etc... all have many great floor plans that are easy to adapt.
Catacombs exist in real life (Paris has a large one) and you can easily get hold of the maps for it. Of course, you can get mines and caves plans as well and expend those.
For example, a quick google search for "tibetan monastery floor plan" got me this temple with floor plans, photos, and explanations. You could use this as a small/medium temple in your world with great ease.
Finally, there are a lot of computer games out there with dungeons: just get one of the strategy books (or walk through) and you should have a plethora of unrealistic dungeons filled with monsters that just wait for the PCs to come and kill them.
In D&D 5e, the daily rate of mounted overland travel is generally the same as on foot, because horses get tired and adventurers carry a lot of heavy equipment.
See the section Special Travel Pace in the DMG (p. 242–243). This section starts:
The rules on travel pace in the Player’s Handbook assume that a group of travelers adopts a pace that, over time, is unaffected by the individual members’ walking speeds. The difference between walking speeds can be significant during combat, but during an overland journey, the difference vanishes as travelers pause to catch their breath, the faster ones wait for the slower ones, and one traveler’s quickness is matched by another traveler’s endurance.
In the same section, the rule is:
- In 1 hour, you can move a number of miles equal to your speed divided by 10.
and then:
- For a fast pace, increase the rate of travel by one-third.
- For a slow pace, multiply the rate by two-thirds.
So an unencumbered horse with a speed of 60 could theoretically travel 6 miles in an hour at a normal pace. At a fast pace (a gallop), 8 miles per hour. That's "twice the usual distance for a fast pace", where "usual" means a creature with a speed of 30. This suggests that a riding horse with no rider, traveling alone, can cover 48 miles per day at a normal pace.
So the rule that "a mounted character can ride at a gallop for about an hour, covering twice the usual distance for a fast pace" seems to exist to allow for mounted travelers covering short distances quickly by using the mount's speed instead of "the usual pace", for up to an hour each day.
So, according to the rules, a traveler on a horse at a normal pace (3 miles per hour) will cover about 24 miles in an 8-hour day. If you make the horse gallop for an hour each day (fast pace for a horse being 8 miles per hour), that range increases to 29 miles. That's within the realm of what you would expect in real life, with a fast horse on good roads in fair weather.
Variant: Encumbrance
If you're using the encumbrance rule, a Riding Horse needs to be carrying less than 80 lbs of rider and equipment to get its full speed of 60. Loaded with between 80 and 160 lbs it has a speed of 50, and carrying between 160 and 480 lbs (its maximum carrying capacity) it has a speed of 30. A 200 lb adventurer in chainmail with a dungeoneering pack, longsword, and shield weighs in at about 325 lbs, so under this rule a horse's travel pace is usually the same as an unencumbered adventurer on foot.
Best Answer
There are some guidelines in the DMG for mapping your world, starting at page 14.
They suggest that for the widest scale (which they call "Mapping a Continent") that you should use about 60 miles per hex. This will make a world a bit smaller than Earth, but still something plenty big for a campaign.
They've also got some other useful guidelines for mapping out terrain, kingdoms and civilizations that might help you.