As a personal trainer, I often come across runners and triathletes that do not include strength and conditioning in their programmes.  They should.

I didn't when I first started out running and was plagued with injuries, nothing too serious, but these could have been avoided if I had included some strength endurance training into my programme.  

As I moved into Triathlon, I realised how important it was.  

You don't need access to a gym, you can do this in your back garden or a local park.   Body weight exercises are a great place to start.