Nooo! You should definitely follow a structured training plan! If you don't know which zone you are in for any particular session, it will give you zero benefit. I mean, imagine if you were supposed to be running in zone 4, but you were actually in zone delta 5 rhesus monkey tennis negative? It would be disastrous! You MUST also read a magazine called "220 triathlon". It looks like a complete bag of sh*t to the untrained eye, but actually that's just a ruse to fool the uninitiated - the pages are secretly glossed with snake oil, which you absorb through your fingertips as you read. Genius! Good look with it all.
On the other hand, you could just follow Barlos' advice. Probably will give the same results.