It is nearly half past five we cannot reach town before dark.
Although this is generally considered a stylistic error, it is occasionally used in literature and may be used as a rhetorical device.
Run on sentences occur when two or more independent clauses are joined without using a coordinating conjunction (i.e. for, and, nor, but, or, yet or so) or correct punctuation (i.e. semicolons, dashes or periods). 
A run-on sentence can be as short as four words - for instance: I drive she walks. In this case, there are two subjects paired with two intransitive verbs. However, with correct punctuation, a writer can assemble multiple independent clauses in a single sentence; in fact, a properly constructed sentence can be extended indefinitely.
There are several ways to correct a run-on sentence.
- Change the comma to a semicolon or dash:
- It is nearly half past five; we cannot reach town before dark.
- It is nearly half past five—we cannot reach town before dark.
- Write the two clauses as two separate sentences (Note: this may disconnect related independent clauses and cause some of the meaning to be lost):
- It is nearly half past five. We cannot reach town before dark.
- Insert a coordinating conjunction with a comma:
- It is nearly half past five, so we cannot reach town before dark.
- It is nearly half past five, and we cannot reach town before dark.
- In British English, there would be no comma before the word and (i.e. It is nearly half past five and we cannot reach town before dark.)
- Make one clause dependent on the other:
- Because it is nearly half past five, we cannot reach town before dark.
- It is nearly half past five, which means we cannot reach town before dark.
Despite the run-on sentence being generally considered grammatically incorrect, there are numerous examples of them being used in literature.
- The short story ""Repent, Harlequin!" Said the Ticktockman" is known for its use of improper grammar, one example of which is a paragraph about jelly beans composed almost entirely of run-on sentences.
- James Joyce's novel Ulysses comprises streams of consciousness that often take the form of long and unpunctuated run-on sentences, particularly the final chapter, "Penelope".
- Examples adapted from the online, public-domain 1918 edition of The Elements of Style.
- Berry, Chris; Brizee, Allen (2006-08-31). "Run-ons – Comma Splices – Fused Sentences". Retrieved 2008-01-24.
- "Run-on Sentences, Comma Splices". Retrieved 2008-01-24.
- Hairston, Maxine; Ruszkiewicz, John J.; Friend, Christy (1998). "The Scott, Foresman Handbook for Writers" (5th ed.). New York: Longman. p. 509.