So i'm 11 hours into Rogue Legacy and I've only defeated the first boss.

Am I doing something wrong?

Currently I'm just trying to grind out stats to overpower everything.

It's rare that I manage to clear out the castle, and if I do so I'm in a low state to enter any new areas? (Which one should I be entering first? The forest?)

I've played 10 hours according to Steam and I still haven't defeated the first boss.

Nah, that is generally how it goes until you learn some of the little exploits. Primarily; play Barbarian Kings, Hokages and Spellswords, and use area spells like crowd or especially flame barrier.

I spent 20+ hours only having beaten the first boss, and then I beat the last three in an hour. Granted I also bought the most expensive, intangible bonuses earlier than later, so I might have been hindering my early growth.

I think it took me about 22 hours to complete the game but it took a long time just to beat that first boss.

You're doing fine. Just keep playing it the same way, and you'll get there. And yep, the forest is the next step.