Working out naturally!

hi everyone,
I'd like to know some peoples opinions on working out without any stimulants.

I've recently got back into training seriously and I've always trained using a stimulant for that explosive feeling, my most recent being gorilla fitness burn bullets and strong coffee.

Over the past couple of weeks I've been getting migraines then was struck down with a virus. The doctor put the migraines down to the stimulants and the virus, well he said my immune system was shot because I'd been pushing my body further than I should have been naturally.

This week I've been training without the stimulants and I am very disheartened. What I thought I could achieve running wise is a complete lie! I can only actually complete half of my run without any stimulants to fuel my workout. I refuse to let myself be beaten though. I am currently using an app for running and I won't progress to the next stage now until I can complete the run im on, without pausing the workout and with no stimulants.

Has anyone else noticed what a huge difference stuff like this makes? I can't believe what a false sense of fitness it gave me. From now on I'll be fueling my workouts with determination and the right diet.