I changed my runtime environment from 5 days to 7 days. I would have expected my MTTD value to get smaller because I am running the system longer. Why did my MTTD value get larger?

The MTTD value is simply the reciprocal of the monthly defect rate. Suppose the expected defect rate is 100 defects found during the last month of phase 3. This is a theoretical monthly rate number based on an average runtime environment. The MTTD for this month is therefore 1 month/100 errors or .01 months between defects. 

Because it is difficult to think in terms of .01 months, you can specify a more intuitive MTTD time unit.  If you select days, then .01 months is the same as .01 * 30 (roughly) or .3 days between defects (assuming you are running 7 days a week). However, if you are running only 5 days a week, then .01 months converts to (.01 * 20) or .2 days.