D&D never was a particularly elegant game for just one player/one DM. There is a word for PCs who take on daring heroics against impossible odds, and that word is dead. It is pretty reliant on the DM to cater heavily towards encounters that have some sort of back door, a lot of diplomacy, allow for stealth, and the like. Otherwise a bad series of die rolling will end the campaign and its back to square one. The one player party take a lot of punishment when luck does not go their way as opposed to a large party which as a whole is nigh indestructible.
Anyhow, there is no single solution so I am taking a multitude of smaller changes as opposed to making huge changes that would not longer make the game D&D. These include a slight buffs for PCs, removal of save or die scenarios, ease of npc inclusion (including animals), and in general a campaign leaning towards story-driven scenarios rather than hack n slash. XP is also included since the default B/X xp will have wonky results with little xp in fair fight but huge boosts for treasure hauls.
For XP I've always liked a method described in 3.0, which is a level gained every 14 encounters. In my 3.x campaigns I use this method purely since it simplifies things. 'Encounter' is not necessarily combat, it could be any relevant scene that has a chance of success, failure or moving the plot along in a major way. I think I will use something similar for Valley of Blue Snails since it should work with one player just as well as a group. Also it won't overly emphasis combat, since a solo player should be pretty damn cautious about unneeded battles. To cater to the B/X curve I will tweak the by-encounter xp gain:
Level 1 - 8 encounters
Level 2 - 10 encounters
Level 3 - 12 encounters
Level 4 - 14 encounters
+1 encounter per-level beyond that
General role-playing and trying wonky stuff will qualify as a encounter. If we happen to spend 4 pages of a thread to buy a bronze-longsword from a particularly hagglesome merchant, so be it. That shall count as an encounter so long as I am rolling for something. As for how this method compares to normal XP gains, I suppose it depends on the DM. I would say this is roughly about right in the games I had - but then again I tend to avoid the sprawling dungeon crawls and opted for more scenario driven games. Ala, Assassin's Knot as opposed to Keep on the Borderlands.
So lets say about 2 pages on a thread per 'encounter' give or take, by the above method to get to level 10 that is around 260 pages. I would say that is about right actually and definitely doable in a years time. If we are ambitious and double that, that would be around the level 'limit' of 15 (not counting dual classing). I doubt that would happen but you never know. This assumes we do a bulletin board thread type of play (I am leaning towards this now).
As a side note, I'm a big fan of crpgs, old and new, so it is a bit odd that I will kill 10,000 Furbogs in WoW over and over and over, yet repetitious combat in a table top is dull for me. Who knows. One mmo that rewarded things by encounter, similar to above is Age of Conan. The start of the game is heavily story driven and differs quite a bit between the classes you play. An assassin likes to stealth, barbarians like to kick in the door, mages like to .. do magic, and so forth. Also you get plenty of xp from replacing the virgin blood with whores blood during a dark ritual as you would from any major combat quest. Nothing wrong with either element, apples and oranges. Too bad Age of Conan went seriously down hill after the starter Tortage story.
edit: speaking of solo adventures.. http://www.kenzerco.com/free_files/KODT_solo_adventure01.pdf