First of all congratulations on getting to 16 miles and undertaking training for your first marathon.
Are you following a canned training plan? (eg Higdon, Galloway, etc)
How many miles per week are you running besides your long run?
How fast are you trying to run your long runs as compared to your target marathon pace?
Do you have a recent race that you've run to estimate your marathon pace from?
How are you handling nutrition and hydration during your long runs?
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