One major thing, obviously traffic is an issue but I found the biggest problem to be **pedestrians **- they waft down the road, head phones on, coffee in hand, reading their text messages and just step out into the road without looking. And, obviously, they can’t hear you coming. Give the pavement a wide berth and watch pedestrians like a hawk.
As others have said - wear bright clothes. I had a great hi-viz, water and windproof jacket which really kept me visible and protected from the elements, plus waterproof gloves. Every serious commuter wore one - you could tell the ‘fair weather’ cyclists by their inappropriate clothing. Flashing lights and a helmet to complete the set.
I try to cycle to work once a week. I live in the UK south of London and my commute (from Surrey to West Sussex) is about 25 miles over mostly country lanes. I’ve got a decent bike and am fairly fit so this takes around an hour and a half. The roads are mostly quiet but the real danger is morons zooming along above the speed limit on these narrow roads. It can get very dark under tree tunnels as well. So like everyone else has said, check your lights. Make sure they are charged and carry spare batteries. It’s a good idea to wear a red blinker on your helmet or backpack so you are visible if you come off your bike.
Check the weather reports - it may be nice on the way in but dreadful on the way home.
Where there is more traffic it’s important to be assertive and ensure you have plenty of space. Keep your distance from larger vehicles - lots of accidents happen when heavy goods vehicles are turning at junctions and can’t see the cyclist coming up alongside.
As it’s a longish ride I will try to have some carbs beforehand like a banana or flapjack.
Finally make sure you lay out everything beforehand packing your rucksack, or make a list. I have forgotten either a towel or shoes on various occasions and it is not much fun finding this out when you get out of the shower at the office.
If you have flexible work hours, take advantage of it to avoid rush hour traffic. An hour makes a big difference.
I don’t have panniers but found a white backpack on amazon which helps visibility versus the typical dark ones.