It is perfectly fine to submit several runs to the same category, while your previous runs are still waiting verification.

By the site rules, runs should be verified by the moderators in 0-21 days (after that the moderators are regarded as inactive). So it is very likely that you improve you runs while previous runs are not accepted yet.

When you improve your run at any category, the older (slower) runs are NOT deleted or removed from the leaderboard, they are marked as "obsolete". You can see obsolete runs on the leaderboard with a relevant filter, and on any user profile page you can see their personal obsolete runs if you check the "show obsolete runs" switch. You should submit everything so you can have a history of your attempts.

Also if you get 2 PB's in a day, you can submit both and usually they'll both get verified.

Furthermore, it is specifically discouraged by site guidelines for game moderators to reject submissions simply because they are obsoleted by newer/better runs. So don't worry about that happening and if it does, call 'em out on it.