Assuming your client is medically cleared to significantly increase his or her training level and is currently running at a moderate pace for a total of 40-60 minutes/week, the first step is to build the fitness base. This is done by progressively increasing exercise volume (frequency and duration) to an initial target of 150 minutes a week, while increasing total weekly mileage or running minutes by no more than 10% to prevent overtraining injuries. Depending on your client’s health and fitness status and responses to training, this will take about 10-14 weeks. I recommend first gradually increasing the duration of the current runs to 30 minutes; then, add a third weekly run. Once your client is running for 30 minutes, 3 times a week, add a fourth weekly run and again build up to 30 minutes, and so on.
Once your client is consistently running 150 minutes a week, it’s time to begin the half-marathon training program. There are many training plans available on the web; Runner’s World is a good resource. You can use a canned plan from a reputable source as a foundation and customize it as needed for your client. Generally, long-distance training programs involve alternating easy and hard runs, with shorter runs during the week and one gradually increasing longer run on the weekend, plus 1-2 rest days and optional days of cross-training. For example, you might include a 3-mile hill run on Monday, followed by an easy 2-miler on Tuesday, a bike ride or swim on Wednesday, a 4-mile run on Thursday, an easy 2-miler on Friday, a moderate 6-mile workout on Saturday, and a rest day on Sunday. Note: every 4 weeks or so, reduce the long run by 2-3 miles to give the mind and body a break. Resume the gradual progression of the long run the following week.
The biggest mistakes I see in half-marathon training are running too much, too soon — and too much, period. People get excited and start running as much as possible, piling on more mileage week after week, until they’re sidelined with an injury. Training smart requires carefully planning each workout to get the most benefits with the least amount of risk. Keeping weekday runs relatively short, undergoing only one longer run each week, gradually increasing weekly time/mileage, and allowing adequate time for rest and recovery helps accomplish that goal.
For more tips, see my previous answer to a half-marathon training question, and check out the resource below