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Yes, it will be too late now. There won't be enough time to get a harvest before cooler temperatures set in. It is best to get these started indoors 6 weeks before the soil remains above about 50 degrees. Once the soil warms up some, you will already have plants to plant outside, rather than starting fresh. This will ensure that you get a harvest before the season's end.

Unfortunately, this can have a few causes.

Sometimes a calcium/magnesium deficiency can appear as rot, and even promote it. This can be corrected, simply, with DOLOMITIC LIME.

For actual cases of rot...

You will have to ensure that the ground does not remain wet from overwatering to completely prevent the problem. Since that may be difficult to do, I would recommend adding WETTABLE SULFUR to the lime treatment. These two together will treat an array of issues, while conditioning the soil.

Otherwise, you should also treat the upper portions of the plant with a fungicide. This article will help: https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm

It appears that your plant is starting to go through its finishing process! This is the time that your cantaloupe will ripen and get themselves ready to pick.

These articles will help:

https://www.gardeningknowhow.com/edible/fruits/cantaloupe/growing-cantaloupe.htm

https://www.gardeningknowhow.com/edible/fruits/cantaloupe/harvesting-cantaloupe.htm

Unfortunately, your photos did not come through. I am unable to see the damage, and area around the plant.

There are many things that can cause this. Low humidity, high temperatures, over and under watering, calcium and magnesium deficiencies, phosphorus deficiencies, and several other factors can cause this.

Sometimes, this can be normal behavior until the plants are mature enough to bear fruit, comfortably.

Here are some articles that will help:

https://www.gardeningknowhow.com/plant-problems/environmental/why-healthy-blooms-fall.htm

https://www.gardeningknowhow.com/edible/fruits/cantaloupe/growing-cantaloupe.htm

https://www.gardeningknowhow.com/edible/fruits/cantaloupe/cutting-back-cantaloupes.htm

https://www.gardeningknowhow.com/edible/fruits/cantaloupe/harvesting-cantaloupe.htm

This article will give you some ideas about how to grow your canteloupes vertically:
https://www.gardeningknowhow.com/edible/fruits/cantaloupe/growing-cantaloupes-vertically.htm

One extension service suggestion is to use phosphorous- and potassium-heavy fertilizer for melons, with a mixture of 5-10-15 or 10-15-20 to enhance growth, flowering and fruiting. Epsom salts will sweeten the fruit a bit, and fish fertilizer, (approximately a pound for every 4-5 square feet) is great, but make sure you don't overfertilize!

I see leaf miner damage, and fungal damage. This is likely due to the leaf miner damage.

Here are two articles that will help:

https://www.gardeningknowhow.com/plant-problems/pests/insects/leaf-miner-control.htm

https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm