Brief Answer:
Skin lightening as advised. continue sun protection.

Detailed Answer:
Hello Mr XXXXXXX and welcome to HCM
As you mentioned, prolonged sun exposure has caused tanning and darkening of your skin.
Firstly we must keep in mind that tanning is a natural mechanism where the skin tries to protect itself from excess sun damage by increasing the skin colour which helps in blocking the UV rays going deep into skin. So any sun exposure will result in darkening. when you are exposed regularly, it becomes a more permanent XXXXXXX which takes much longer to go back to normal.
Other factors like pollution , smoke, aging etc can add to it.
Sunscreens only protect the skin from sun damage and prevent tanning to an extent. They do not lighten the skin colour. It is necessary for you to use a good sun screen with SPF around 30 , regularly when outdoors, even for short periods. Apply generously on all exposed parts.
When you travel, try to wear light, full covered clothing and footwear.
Especially when you are on skin lightening treatment, sunscreen must be used.
Complete tanning cannot be fully prevented as long as you are exposed to direct sun.

As for lightening treatments, chemical peels can be used for face, neck etc as advised by dermatologist. Make sure your skin is not sensitive or allergic to it. They require a few sittings, depending on the improvement.
On a regular basis, you can use lightening soaps and facewashes like Dermadew Lite. Glycolic acid facewashes like AHA glow, or lightening face washes for men.
Home remedies like lemon, honey, tomato etc can be tried.
Over the body, skin lightening lotions may be used after bath, like Nivea.
Most importantly, pay attention to prevent further XXXXXXX

Other cosmetic procedures such as LASER can be tried. A newer method of Glutathione + Vit C pills/injections are now available which work in the body and have shown good results. This also requires multiple sittings and maintenance. Consult a reputed dermatologist regarding details and cost.

Hope this helps you. Do remember that it will take tme to return to previous skin colour. Eat well and have a nutritious diet with enough fluids.

Feel free to get back to me wtih any doubts/ clarifications.

---