At risk of starting some kind of off-topic flame war, I'll say this. I've played on MUDs for a long time: years in fact. (For you young whipper-snappers, MUDs are like EverQuest without the graphics. Yes, I do use a cane; thanks for asking.) Now a MUD is similar enough in its tactical aspects to a D&D solo campaign that I feel qualified to state the following with conviction:
A solo adventurer can succeed as any class, but each must approach things differently, playing to his strengths.
Are you a fighter? Then do what you do best: fight. Take those combat feats and a big weapon and dish out damage faster than your enemies can soak it up. Use the best armor you can afford and your high HP to outlast people in the fight.
Are you a cleric? (You've said you don't want to be, but what the heck: free advice.) You can fight moderately well, and your spells give you a serious advantage. Even a single cure light wounds swings the tide in your favor. Plus, you can predict the future, buff yourself, make food and water, summon creatures to do your bidding, and enable fast travel. Nice. (And IMHO, clerics are the best solo adventurers of them all.)
Are you a wizard? Then like the cleric, your spells give you an important edge. But you must avoid combat at all costs. Use ranged and area spells to good effect. Summon monsters to fight for you or to cover your escape. Once you get invisibility, scout out the area and only interact with those you choose. Focus on survival, because at higher levels, you rock.
Are you a rogue? Then be a rogue: hide, move silently, sneak attack. Run away from tough foes. Ambush people. Disarm traps, or better yet, set them for your enemies. Pilfer things without getting into fights at all. Be the unseen shadow that strikes without warning.
Play to your strengths. Specialize. Do your class's thing with ruthless effeciency and utter singlemindedness. That is how to succeed as a solo adventurer.