Plantar fasciitis is a frustrating condition, because treatment involves avoiding activities that irritate the involved tissues — typically running, walking, jumping, and other activities that fitness enthusiasts love to do. And, depending on the severity of your symptoms and how effective your treatment is, you could be convalescing for months.
Your doctor is a better judge of this, but I’d say it’s unlikely you’ll be running a half-marathon in five months. If you do too much, too soon, you’ll increase your odds of experiencing a setback; and that’ll make your recovery timetable even longer. If you skimp on training and run the race anyway, you risk further injury.
Stay active while you’re in recovery. Include non-weight-bearing activities like cycling and swimming in your training plan to maintain your cardiovascular fitness; continue strength-training. These activities will help keep you in shape, but they won’t adequately prepare you for a half-marathon. To train for a running event, there’s no substitute for running.
If you haven’t already, seek the advice of a sports medicine physician or podiatrist. Be patient, and allow plenty of time for the healing process. When you’re back on your feet, there’ll be plenty of half-marathons to train for.