How do you make a fast paced story last 80/90k words without killing the pace? I've heard people mention sub-plots and description, but don't they slow the whole thing down? I just don't know how it's done! Gah!
like others said, find thrillers you like and analyze them.
for mine, that WAS a problem....mc spends a fair amount of time healing in and out of a hospital and planning his kills, which is relatively boring.
On thinking about him though, there are a lot of things going on--he's hardening, becoming more distant. A personal friend is trying to reach out to him, and he's trying to keep his distance without also tipping folks off something is wrong (he's blaming his head injury for loss of memory to buy time). He's trying to avoid and evade a sheriff who has his suspicions. The guys he's after are unravelling all on their own. He needs to get a gun, and does, and in the process learns abit about what he's becoming.....
there was plenty to fill the middle, and much of it wasn't explosions and car chases. Had it been, it probably would have been more 2-dimensional for it. You want to invest in character at some point, and you can't just completely drop the thriller part and then start it back up 200 pages later, if you read the thrillers you liked, very often there is an initial conflict scene, a climax, and maybe a handful of events in the middle, but also a lot of in-between where we get to meet the character, and learn he likes sushi and cats or whatever, because he isn't so damn busy being shot at.