Hola! I see you're Spanish and it's true that you say something can be "pesado". In English we don't use "pesado" in the same way. We use more specific adjectives for this, e.g. boring, long, tiring...
The journey was really long / boring / tiring
For something more generic and more colloquial (like the word "pesado") you can say in UK English: "a pain"/"a pain in the neck"/"nightmare"
- The journey was a pain in the neck! / The journey was a nightmare!
- Because we got delayed for 5 hours at the airport and then when we got to the car, we saw we had run out of petrol
"heavy" solo se usa para objetos que pesan mucho.
Espero que te sirva :)