Unless I am entirely mistaken, only half of the areas need to be completed to finish the game. The game is VERY difficult imho. There's a certain amount of leveling up (by points and collecting blue/red landers) that is helpful, and maximizing item collection/usage would be quite tricky.
As noted, the game is fairly linear - the introductory stage, the 10 stages that are required to complete the game, and the final stage are all autoscrolling shooter levels, as well as the 10 extraneous shooter stages which serve only to provide (often very helpful) items. In between the shooter stages is a legend of zelda-style overworld, with new areas being opened up as the key stages are passed (granting access keys), and minibosses (which provide attack or defense upgrades, as well as subweapons (which have multiple levels of potency).
The game is both faster paced and slower paced than Gradius... the ship has a fixed movement speed, but the stages scroll more rapidly than the stages in gradius - someimes drastically so. There are no walls to crash into, and the only hazards come from enemies, which can be dependant on the terrain (and thus approach at a rate relative the speed at which the stage is scrolling) or independent from the terrain (which tend to be the same general speed no matter what stage you are on.
This WOULD be a lengthy speedrun; I estimate no less than 40 minutes. However I believe it would be more interesting to watch than say, the Gradius runs. The novelty of seeing Vic Viper spazzing around the screen wears off rather quickly, and there is a wide variety of weapons and enemies in TGL that can be used to keep the action interesting.