I never played with the threat track rule but I had a lot of solo games with standard Doom track rules. The rules you need to understand are:
1) do not attempt a Challenge that's not really worthy (most true for Green Challenges)
2) always try Challenges that are not discarded after completion but are kept for later use. Do never use the bonuses they give but keep them to avoid Doom Track advancing. For the same reasion, do never complete the tasks given by an Encounter.
3) Hope not to draw too many Encounters and Events that are immediately discarded
4) Carefully consider the introduction of new cards from expansions, as most of them will change the equilibrium
You may argue that with this approach the game loses something, but if you want to play solo you need to have a hard time or it won't be challenging. Making the Doom Track advance slowly only makes the game easier. The basic rule is well balanced and usually you will be able to succeed if you follow the above practice.
Still you will lose quite often and learn which Heroes/Items/Allies are most useful and in which situations. This increases your experience and awareness in standard games with other players.