The weight of what you are carrying right now is real, and it runs through your body, your checkbook, and your mind all at once. It is not weakness to admit you have come to the end of your own strength for field work. In fact, that is often the precise place where the Lord begins to show what His strength can do. When we look within ourselves for the secret reserve of energy to push through, we eventually hit a wall. But when we look to Him as the source, we find that His supply does not run dry. He will not let you down.
You asked for wisdom, and that was the perfect thing to request. So many people in your situation would have asked only for an easier schedule or a bigger paycheck, but you put wisdom first. There is a reason Scripture calls it the principal thing. A promotion with more money but without wisdom is a trap. But when you have the wisdom that comes from above, you get something far better than rubies. That wisdom is pure, peaceable, and gentle. It is not the calculating, envious striving of the office politics around you. It is the practical skill that knows how to build a house, or in your case, how to build a career in a new department.
When you feel that your body cannot do what it once did, and the mental strain is clouding your thinking, remember that God gives wisdom generously and without scolding you for your past mistakes. He does not upbraid you for the moments you forgot to be thankful. He just pours out more of His Spirit of understanding. Christ Himself is made unto us wisdom. In Him are hidden all the treasures of it. You are not just asking for a business strategy; you are asking to know His way forward. That is the wisdom that stood beside Him when He established the heavens, and that same wisdom is available to direct your path through this promotion process.
Do not let shame take root regarding your physical limitations. God often uses the very things that weaken our grip on this world to cause us to take a stronger grip on Him. He does not require you to chop wood with a dull axe until you break. Wisdom shows you how to sharpen the edge, or in your case, how to transition to a role where the gifts of your mind and the diligence of your spirit can be used for His glory without destroying your body. The work He has for you in an office setting may be just as sacred as any other labor if your heart is willing. He is the one who stirs up willing hearts and fills people with the knowledge to do all manner of work.
Trust that as you surrender your career into His hands, He is preparing a place for you that aligns with His perfect knowledge of your frame. Your faith stands in the power of God, not in the wisdom of men or the fluctuating strength of your own flesh. Keep asking for that understanding heart. The fear of the Lord is where it starts, and you are already standing right there at the starting line.