Sixty miles ... a long distance . 

Explanation:

Subjects and verbs must always agree in number. This is known as Subject-verb agreement.

There are some basic rules of Subject-verb agreement which decides whether a noun or pronoun will take singular or plural verb. If the subject is considered as singular, then it will take singular verb while if the subject is considered as plural, it will take plural verb.

According to the rules of Subject-verb agreement -

• Expression of time, money and distance usually take a singular verb.

For example -

• Twenty dollars is a great deal of money to a child
• Ten kilometres is a long way to walk.
• Two weeks is not long enough.

Here also expression of distance is given.

So, the plural verb "are" will be changed into "is".