Saturday is my go-get-em day where I can run for longer periods of time because the specters of homework and internships are a little more removed.
I started on my run in my neighborhood, planning on running 3 miles to the best of my ability and walking another 2 with the goal of eventually building up to where I can just run 5 miles. That all came to a rather abrupt halt when I only made it to the corner of my street and my lungs felt like they were on fire. I made it a grand .65 miles before it hard to breathe at all.
My fiance is far more experienced than I am when it comes to training for a half marathon. He told me all about the dreaded "off" days. The ones where you just can't get into your rhythm and no matter how often you run it's going to happen. But when you're faced with the off day challenge don't give up. Running may have been out for the day but I could still walk so I went for a beautiful 2 mile stroll along the trail behind our house and did some strength training. Walking and doing strengthening exercises shouldn't be underestimated. Remember, every little bit helps.
Off days can also be caused by a more corporal factor- what you eat. I discovered that even though I was drinking high volumes of water my body wasn't absorbing it. I took a look back at my diet and found a major component was missing- salt. I found that Runners World has a great section with recipes and tips for how to stay hydrated and pack the nutrients you need into your diet. It's these little things that make a difference and by being conscious of what are are putting in your body you'll be ready to get up and go!