From my experience, as long as you got stamina, it doesn't matter with what kind of map you start.
I persoanally doctor around on my first two maps since more than a year now - and yes, they're an AoS and a TD, both involving lots and lots of custom units, hundreds of tiring tooltipps nobody even cares to read and even more triggers
My way to not tire myself too much out is to swap between the 2 projects once I reach a point where I feel I'm stuck too much since I'm out of ideas / my current skill level is too low.
A good practice is to look around in the world editor help zone and request thread, searching for interesting request I think I can handle and work on them - it's both helping others, learning new (or practicing new stuff) and challenging, tohugh not too much over the top.
In addition, there are tons of tutorials out there, helping with stuff creation and explaning a bunch. If that isn't enough to satisfy your thirst for knowledge, search the hive for some spellpacks in your preferred language (either GUI or JASS), save them, open them, try to understand the triggers and you'll soon wnat to implement something similar in your own map xD
The road of a triggerer is long and frustrating, but not lonely - we're all here to help with hints, code and feedback
Just a quick note about your worker idea: that map only needs some custom units and defeat/win conditions, unless you wanna add some cool abilities to your towers. Oh, and maybe you'll need a trigger that either allows human players to give bounty or any other source of income
I hope you're trying a little bit more than just that - it's frustrating having to write lots of tooltipps and in the end these tooltipps being 90% of the work done ='(
(one of the reason's I'm taking a break from my td currently)