You should have enough fuel...
In the 'stock' game run at around 9.5 knots while surfaced for best range instead of ahead standard.
You can do this by clicking the tab beneath the telegraph it will change from notches to actual speeds.
Even an hour or two at flank is dicey so plan your attacks well ahead of time.
The 'extending range by running submerged on batteries' has been the subject of much debate. In real life it would not be possible due to the energy spent recharing the batterys. However I personally feel it does extend range. As a standard operating procedure my boats tend to spend a lot of their time submerged in the daylight hours.
One more tip, if you get a 'patrol area' mission just sail to the area and cut the engines and drift around for the patrol duration, diving when aircraft are spotted and only using engines when prosecuting a contact.
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