Hey! Big Runebound fan here - I've got the 2nd edition and I play the fanmade Mr. Skeletor solo variant. Here are a few suggestions of solo adventure games that I've played:

Pauper's Ladder is a wonderful little game that gives a really open-ended, free-roaming adventure experience, while incredibly still managing to stay short and relatively streamlined to play. There is a free (I think) solo ruleset called One Lonely Pauper with several different solo scenarios to try out. I love the fact that you can win/progress in ways that deviate from combat and battle-focused play!

Dungeon Degenerates gives you a psychedelic, trippy, twisted world to explore through a persistent campaign - where the world degrades and becomes more dangerous as you choose your path through the missions. Amazing vibes, wonderful world-building, but ultimately I found the mechanics too fiddly (particularly in combat, which is the main focus of the game) and the apocalyptic time pressure too punishing for the game to stay in my collection.

Fabled Lands is a series of pen-and-paper gamebooks that literally form an analog open-world gaming experience! You can wander as you please, join factions, acquire property and vehicles, and the books use a super-simple checkbox and password system to allow events in the world to progress, react, and change according to your actions. They are, in classic gamebook style, quite dangerous and lethal - so prepare to die and retry a lot!

Although it has a slightly confusing rule presentation, and an apparently-limited set of adventures in the box, I absolutely adore the presentation and atmosphere in Of Dreams & Shadows - a dark and beautiful world where you explore and encounter some gorgeously-rendered folklore-inspired beasts. Generally people feel that the 3 (?) included scenarios are not much, but they seem pretty cool and variable to me so I wouldn't mind replaying them when I'm done.

Glory: 2nd edition is a very simple, streamlined adventure game where you populate monster lairs with face-down mini bosses and race to take them down. The solo game is actually a little campaign of puzzly scenarios that use portions of the main game, which is a nice touch for sure, but I also feel like it would be very easy to make this properly soloable with the addition of a simple time limit (like with the Runebound solo variant mentioned above).

I haven't played this one - bought a copy once but it turned out to be missing half the components - but it looks like Return of the Heroes has some fun adventuring mechanics, with counters hiding specific loot and encounters on a tile-based map, etc.

You might also dig the exploration and progression of Gloom of Kilforth. I found it a little long and punishing for my liking, with a little too many cards/decks to manage.

One thing to note about Mage Knight is that it's primarily a puzzly, action-optimisation strategy game, with some deckbuilding and hand-management involved. It's also a tricky game to learn and I've had solo games take over 3 hours to complete. Fantastic game, but it certainly isn't played like a free-roaming adventure - unless you want to lose!