I am a beginner over 40 who picked up running this past year and did my first half marathon yesterday at 1 hour 48 min. I have never been a distant runner and had problem running even 2 miles 3 years ago. I want to run a convincing under 4 hour marathon by next November. I understand the key is to put in more miles per week (I am only at around 16 miles per week) -- but I am wary of injury. Is it easier on my joints if I run on a treadmill?